Raman light scattering has been successfully used in critical care situations to continuously monitor a patient's respiratory gases. This technique is based on the effect which occurs when monochromatic light interacts with vibrational/rotational modes of gas molecules to produce scattered light which is frequency shifted from that of the incident radiation by an amount corresponding to the vibrational/rotational energies of the scattering gas molecules. If the incident light photon loses energy in the collision, it is re-emitted as scattered light with lower energy and consequently lower frequency than the incident photon. In a similar manner, if the incident photon gains energy in the collision, it is re-emitted as scattered light with higher energy and higher frequency than the incident photon. Since these energy shifts are species-specific, analysis of the various frequency components present in the Raman scattering spectrum of a sample provides chemical identification of the gases present in the scattering volume. The intensity of the various frequency components, i.e., Raman spectral lines, provides quantification of the gases present, providing suitable calibrations have been made. In this manner, Raman light scattering can be employed to determine the identity and quantity of various respiratory and anesthetic gases present in a patient's breath in operating room and intensive care situations.
Systems developed for analysis of gases in critical care situations utilizing Raman scattering typically employ gas cells which contain a sample of the patient's respiratory gas to be analyzed. One such system is described in U.S. Pat. No. 4,784,486, entitled "MULTI-CHANNEL MOLECULAR GAS ANALYSIS BY LASER-ACTIVATED RAMAN LIGHT SCATTERING", issued to Van Wagenen et al. The gas cell is located either within the resonant cavity of a laser or outside the cavity. In an intracavity system, such as that described by Van Wagenen, a laser beam is directed through a resonant cavity such that it intercepts a respiratory gas sample within a gas cell. An end mirror located at one end of the resonant cavity redirects light incident from a plasma discharge tube back through the resonant cavity, where it again passes through the gas cell and back into the plasma discharge tube. A Brewster prism may be mounted near the end mirror to select the desired wavelength and polarization state of the lasing light. The end mirror and Brewster prism are both mounted on one or more plates of an alignment assembly. Raman scattered light from the gas analysis region within the gas cell is collected by collection optics and directed through one or more interference filters or other means of wavelength discrimination. The collection optics and interference filters and possibly focusing optics in turn transmit the Raman scattered light to appropriate detectors for quantifying each specific Raman signal and thus each specific gas comprising the respiratory gas sample.
Intracavity systems possess the advantage that they achieve a much greater Raman scattering intensity than systems in which the Raman scattering occurs outside of the laser resonant cavity. This greater intensity is a result of the fact that a laser beam transiting an intracavity arrangement propagates through the gas sample many times, with a correspondingly higher time-integrated intensity of Raman scattered light being collected from the gas sample. In contrast, an external arrangement of the gas cell may allow the laser beam fewer passes through the gas sample. While intracavity systems benefit from a greater Raman signal strength than do systems having the gas cell located outside the laser resonant cavity, the resonator optics must be positioned with extreme accuracy for this advantage to be realized, since the multiple reflection of the laser beam within the cavity magnifies any misalignment of the cavity end mirror. Similarly, alignment and optimal component performance is critical for external resonant systems. Consequently, the cavity end mirror, the Brewster prism (if present), and the central axis of the laser plasma tube must all be aligned almost perfectly with respect to each other at all times during operation of the gas analysis system. Additionally, transmission of the laser beam through windows, lenses, prisms, etc. and reflection of the laser beam from mirrors, etc. must be maintained at an optimal level or system performance will degrade due to loss of laser power.
In the intracavity gas cell systems discussed above, windows are commonly provided on either end of the gas cell to protect surrounding optical elements and filters from contaminants which may be present in the gas sample. The windows further serve to confine the gas sample within the gas cell, thereby minimizing the volume of the sample and thus improving the detector's response time. In some systems, the gas cell windows are oriented at Brewster's angle to select and improve the transmission of a particular polarization state of light passing through the sample. In this manner, optical losses in the laser beam which passes through the cell are minimized. However, the gas sample, in combination with particulates often carried with the sample, may contaminate the cell windows and degrade the performance of the system. This contamination may result in undesirable light absorption and/or scattering with a consequent decrease in the laser power circulating through the sample cell. If untreated, this contamination will eventually cause the system to cease to function properly.
The problem of window and cavity optics contamination has been partially solved by providing an air dam around the optics of the laser system to shield the optics from contaminated samples. Systems for providing such an air dam are disclosed in U.S. Pat. No. 5,135,304, entitled "GAS ANALYSIS SYSTEM HAVING BUFFER GAS INPUTS TO PROTECT ASSOCIATED OPTICAL ELEMENTS", issued to Miles et al. and U.S. Pat. No. 5,153,671, entitled "GAS ANALYSIS SYSTEM HAVING BUFFER GAS INPUTS TO PROTECT ASSOCIATED OPTICAL ELEMENTS", issued to Miles. In intracavity systems such as those disclosed in U.S. Pat. No. 5,135,304, the sample of gas to be analyzed is injected near the center of the array of detectors. Simultaneously, a buffer gas such as nitrogen or filtered air is injected on the sides of the analyzer cavity. Both gas streams are exhausted at an intermediate point. This system advantageously provides a pure gas sample near the detectors while protecting the optics of the resonant cavity from contamination carried by the sample gas.
In spite of the advances made in protecting the resonant cavity optics from contamination, individual portions of the resonant cavity, including the end mirrors, gas cell windows, lenses, prisms, laser plasma tube, etc. must still occasionally be disassembled and cleaned of contamination, repaired or replaced. At such times, the optical elements are disassembled and repaired or cleaned, then reassembled. The high degree of precision required of the optical alignment of the system, including the alignment of the end mirror, Brewster prism (if present), and plasma discharge tube, renders field repairs difficult. Thus, most repairs are presently made at the factory where the system can be placed on an optical bench or fixture for precision alignment of the components.
A decision to repair resonant cavity optics is typically made based: (1) on the number of hours such optics have been in service; (2) on some periodic service interval; or (3) when a human operator notices degradation in the functional performance of the unit. Decisions to maintain resonant cavity optics on such bases may result in optical performance degradation to the extent that the cavity no longer performs at an optimal level. Moreover, the lack of a quantifiable basis on which to predicate maintenance of the cavity optics also may result in too frequent, and hence, unnecessary maintenance. A more cost effective basis for justifying such maintenance would be based on some quantifiable performance criteria of the cavity. Therefore, a need exists for a method and system for monitoring the level of particulate contamination on the surfaces of the optical elements in the cavity.
As described above, progress has been made in preventing contamination of the optical elements in Raman gas analyzer systems. However, in present systems, the most prevalent means for monitoring the system for contamination is to monitor the total laser power circulating in the cavity. Since this is a relatively large number compared to the losses initially caused by contamination, it does not provide a very sensitive measurement for contamination and it does not provide information which is useful for localizing the source of contamination. Furthermore, other factors affect total laser power circulating in the cavity, including: contamination of any one of the optical surfaces; misalignment of the optical components; occlusion of the intracavity space; increased Raleigh scattering by gas constituents in the measurement cell; or malfunction of the laser gain medium. Heretofore, no systems have incorporated means specifically designed for detecting and measuring contamination at selected locations in the system. Thus, contamination of the laser optics in the laser cavity from dust, dirt, particulate matter or film build up is still a major problem which can make it difficult to sustain reliable lasing in an unsealed laser resonator.